1. Field of Invention
This invention describes methods of obtaining position of incidence information from solid state devices, such as avalanche photodiodes, without introducing any dead space to the detector""s active area.
2. Discussion of Prior Art
Many applications in science and industry require detectors that are capable of reporting time and position of incidence information for discrete quantum units of radiation such as single photons and beta particles. A single photon is understood to be a unit of radiation with an energy described by E=hc/xcex, where xcex is the wavelength of the radiation. In some cases it is most expedient to convert a high energy photon into a group of multiple lower energy photons and then detect the group of lower energy photons as a single event corresponding to the lower energy photons. This is typically achieved using fluorescent materials such as scintillators.
Detectors for these applications will ideally have an output that gives a rapid position sensitive readout with a good signal to noise ratio. In order to achieve a good signal to noise ratio, it is beneficial for the detector to have internal gain. The detector should also have good detection efficiency over a large active area and a wide dynamic range. Furthermore, the active area should cover a significant portion of the detector""s physical footprint and allow for efficient tiling to cover areas greater than the practical size of a discrete detector. In some applications, it is desirable for the detector to be capable of operating effectively in a high magnetic field. It is also beneficial if the detector has low power requirements, especially for applications that require many detector elements. A number of technologies have been developed in an effort to satisfy these requirements. These technologies fall into two main categories: vacuum tube detectors and solid state detectors.
Vacuum Tube Detectors
Vacuum tube detectors include photomultiplier tubes, image intensifiers, and imaging photon detectors. These detectors have a photocathode that converts incident radiation outside the detector envelope into electrons inside the detector envelope. Electrons from the photocathode are then amplified inside the detector envelope, typically using a system of dynodes or microchannel plates that confine the amplification process to remain spatially centered about the position at which the electrons originated from the photocathode. The bundles of electrons resulting from the amplification process are then collected on an anode structure that can provide a position sensitive readout, and the position of the incident radiation is then determined from this readout.
Vacuum tube detectors can achieve gains in excess of 106 with relative ease, and can provide sub-nanosecond readout. However, they are limited by the quantum efficiency of the photocathode material, which in practice is typically in the range of 10-20%. In addition, the input window on which the photocathode is formed is generally made of glass or a fiber optic faceplate that is a few millimeters thick. Both methods introduce optical losses when the detector is used with proximity-focused scintillator arrays. Detectors that use microchannel plate structures for internal amplification suffer from a localized dead time on the order of 10-100 milliseconds, which severely limits the realizable dynamic range of the detector for detecting sequential pulses of radiation. Vacuum tube detectors are also frequently constructed in a round enclosure, which is inefficient for tiling to cover large areas. Furthermore, magnetic fields that are not parallel to the electron transit path inside the vacuum enclosure will always cause geometric distortion in a position sensitive readout and may affect gain as well.
Solid State Detectors
There are two main types of solid state detectors that are used in the radiation detection applications described above: photodiodes and avalanche photodiodes (APDs). The fundamental difference between these two types of detectors is that avalanche photodiodes have internal gain, while photodiodes have no gain. This makes APDs a better choice than photodiodes in applications where small signals with low background must be detected with wide bandwidth at high frequencies. Positron Emission Tomography (PET) is a classic example of this type of application, where the timing coincidence of individually detected gamma rays must be measured to within a few nanoseconds while maintaining good energy resolution and high signal throughput. Similar applications exist in high energy physics, LIDAR, and LADAR.
Owen (xe2x80x9cOne and Two Dimensional Position Sensing Semiconductor Detectorsxe2x80x9d, IEEE Trans. Nucl. Sci. NS-15, p.290+, 1968), Kelly (xe2x80x9cLateral-Effect Photodiodesxe2x80x9d, Laser Focus, Mar. 1976, pp. 38-40) Kurasawa (xe2x80x9cAn Application of PSD to Measurement of Positionxe2x80x9d, Precision Instrument, Vol 51, No. 4, 1985, pp. 730-737) and others have shown methods of obtaining position sensitive information from solid state detectors with no internal gain. A number of companies including Hamamatsu, UDT, and Silicon Sensor sell xe2x80x98lateral effectxe2x80x99 position sensing photodiode products that use similar methods. However, because they are photodiodes that have no internal gain, all of these detectors are limited to applications that have relatively low bandwidth requirements and a relatively high background when compared to what is possible with avalanche photodiodes.
An APD is a semiconductor device that is constructed in such a way that a large electric field can be created inside the semiconductor material with a very low leakage current. Any free carriers that enter the electric field region will be accelerated out of it. If the size of the electric field region is large relative to the mean fire path of the carriers, then there is a high probability that a free carrier will gain enough energy to liberate other carriers in the space charge region, which will in turn be accelerated. This avalanche effect continues until the free carriers get accelerated out of the space charge region and either recombine or are extracted from the device. The device is designed such that when an electron-hole pair is created in the top layer, a charged carrier will drift into the high field region of the device and experience avalanche multiplication. The avalanche process gives APDs internal gain, which is very useful for detecting low levels of electromagnetic radiation.
There are a number of reasons why the prior art methods for extracting position sensitive information from photodiodes cannot be directly extended to work with APDs. Before considering how to obtain position sensitive information, however, it is important to recognize that substantially different approaches must be used to design and fabricate a non-position sensitive APD as compared to a non-position sensitive photodiode with the same active area. This is because the internal fields in APDs are much higher than the internal fields in photodiodes, so a field spreading structure is required to avoid edge breakdown when bias is applied to an APD. The details of these methods are well known to those skilled in the art.
The design of a position sensitive APD must give special consideration to the placement of contacts on the device in order to avoid electrical interaction with the field spreading structure. The contact method also affects the package design, which can in turn affect the usability of the detector in tiling applications. In addition, while photodiodes can receive uniform surface treatments to achieve a position sensitive readout, most surface treatments will need to be modified in order to be compatible with the field spreading structure in an APD. Furthermore, it can be advantageous to extract position sensitive information from the majority carrier signal on the cathode in order to avoid modifying the anode structure in ways that could significantly affect the sensitivity or response uniformity of the device. If position determining signals are only extracted from the cathode of the device, then only one side of the device is used to produce a position sensitive signal, whereas in many position sensitive photodiode configurations both sides of the device are used without substantially affecting the sensitivity or response uniformity of the device.
The ability to fabricate commercially viable large area, high gain avalanche photodiodes is a fairly recent development. The prior art for extracting high resolution position sensitive information from large area avalanche photodiodes consists of creating an array of discrete pixels on a monolithic device (for example, Huth U.S. Pat. No. 5,021,854; Dabrowski U.S. Pat. No. 5,757,057 and U.S. Pat. No. 6,111,299, Ishaque U.S. Pat. No. 5,500,376, Gramsch et.al. xe2x80x9cHigh density avalanche photodiode array,xe2x80x9d Proc. SPIE Vol. 2022, October 1993, p. 111-119). This prior art appears to indicate a preference for forming discrete pixel boundaries in order to limit charge spreading inside the device during the gain process so the signal can be read out using one contact. The physical location of the pixel then determines the position of the signal, with the physical size of the pixel determining the spatial resolution of the device. Contrary to this prior art, the present invention uses charge spreading in the device as a beneficial mechanism for obtaining position sensitive information, rather than as a problem that should be minimized. The present invention can achieve sub-millimeter spatial resolution over a large area using a small number of amplifier channels; typically 2 channels for a one dimensional measurement and 4 channels for a two dimensional measurement. By capitalizing on the charge spreading characteristic of large area APDs that was previously considered undesirable for obtaining position resolution, the inventors have been able to develop the methods disclosed in this invention for obtaining position sensitive information from a solid state detector with internal gain.
While the prior art approach of building an array of pixels to capture position information has benefits for certain applications, there will always be either some degree of cross-talk between adjacent pixels, or else some dead space in between the pixels. The problem of cross-talk between pixels can significantly complicate the signal readout, especially when energy resolution of the signal is important, and reducing the pixel size to improve resolution tends to increase cross talk problems. Various approaches presented in prior art that minimize cross-talk between pixels introduce dead space between the pixels. As pixel size is decreased to improve spatial resolution, the ratio of active area to physical device area decreases, which can significantly reduce the amount of signal collected, which adversely affects signal to noise ratio as well as energy resolution. In addition, the number of electrical connections to the device increases in proportion to the square of the decrease in pixel size. The risk that fabricated devices will contain or develop dead or poorly functioning pixels adversely affects the manufacturing process yield as well as the value of the manufactured product. Furthermore, as the number of pixels is increased, the complexity and cost of the readout electronics also increases, especially in applications such as PET where coincidence determinations must be made using signals that extend over a large number of pixels.
Prior art methods for determining position of incidence with high resolution over an extended area focus on determining which element in an array contains the desired signal. In contrast to prior art, positions in the present invention are preferably determined by implementing a calculation based on the relative amplitudes of a plurality of signals measured at substantially the same time. This is a significant improvement over prior art because a small number of preamplifier channels can be used to read out position-determining signals from a large active area with high resolution, and a single amplifier channel can be used to provide a fast timing signal for coincidence detection of the signal from any position within said area.
In comparison with vacuum tube devices, solid state devices immediately overcome a number of disadvantages. The quantum efficiency of APDs in practice is typically in the range of 40-60%, and can exceed 70%. This higher quantum efficiency relative to vacuum tube devices often more than compensates for the higher excess noise of APDs. The detection of radiation by an APD occurs within less than a micron of the physical surface of the device, so proximity focusing to scintillator arrays or phosphors is very efficient. The response time of large area APDs is typically on the order of a few nanoseconds, which is comparable to many vacuum tube detectors and more than adequate for many radiation detection applications. Furthermore, the internal gain mechanism in APDs does not introduce a localized dead time that would limit dynamic range for detecting sequential events at the same position of incidence in the same way that microchannel plate based vacuum detectors are limited.
APDs can be manufactured at low cost using highly scalable manufacturing processes, which makes it possible to achieve a lower cost per unit of detector active area relative to vacuum tube detectors. APDs are very compact and light weight, and can also easily be fabricated in a rectangular format with a high active area to device footprint ratio, which makes them very well suited to applications requiring efficient tiling. The power requirement per unit of active area for APDs is generally less than for vacuum tube detectors, primarily because they can be operated without the voltage divider circuit that is required for proper biasing of the amplifying elements in vacuum tube devices. Finally, APDs are orders of magnitude less susceptible to geometric distortion of a position sensitive readout due to transverse magnetic fields, primarily because the electron transit path is much shorter, and also because the Hall effect will result in a compensating electric field being set up inside the device that tends to cancel the effect of the magnetic field.
The approaches presented here for obtaining position sensitive information from solid state devices with internal gain offer a number of important advantages over prior art in terms of performance, ease of use, and manufacturability.
Our invention consists of a special readout technique that makes it possible to obtain spatial information from within a continuous active area of a solid state detector with internal gain. Since the avalanche event in a solid state device begins at a distinct location inside the semiconductor material, the propagation of the avalanche inside the device is physically centered about the point of initiation as shown in FIG. 1. Contrary to the teaching of prior art, we found that it is possible, and in some cases preferable, to determine the location of that point using position-dependent charge separation techniques similar to those used in other position sensitive detectors. Such techniques are illustrated in FIGS. 3 and 4 and include, but are not limited to, a resistive cathode sheet with one or more contacts, or one or more patterned cathode contacts. A cathode of an APD is understood to be a contact of the device that has a negative potential relative to the anode when the device is forward biased.
This method offers a simple fabrication process, an easy readout approach even at very high effective pixel densities, and no dead space over the entire active area. This method also makes it easy to implement contact patterns that give a non-rectangular position readout as shown in FIGS. 3C and 4B.
Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description.